Natural Ventilation Design

Figure 2 shows the location of the six-story (20 meters) demonstration building and its surroundings in Beijing City Garden, Beijing. There is a long, mid-rise building, 40 meters high, to the north and low-rise six-story buildings to the east and south. A wide street runs along the west of the demonstration building. Our design in this case focused on natural ventilation.

The design of natural ventilation in the demonstration building required data on indoor and outdoor airflow distributions. The wind rose in Beijing, as shown in Figure 3, indicated that in the summer the prevailing wind is from north and south. The corresponding mean wind speed is 1.9 m/s. Figure 4 breaks the reference weather data in Beijing into different comfort categories. Although heating is an important issue in Beijing, buildings there also require cooling from June through August. However, the figure illustrates that the period of comfort can be increased substantially, and the level of mechanical cooling reduced, if mean air speed in the building is at least 2 m/s through natural ventilation or internal overhead fans. Indeed, air-conditioning may not be necessary in Beijing if natural ventilation is combined with night cooling. Excess wind speed, however, is detrimental, causing discomfort to pedestrians. In Beijing, winter winds around some buildings can reach speeds of 14 m/s. Considering the low air temperature in the

winter, the chilling effect of this wind prevents pedestrians from walking comfortably and safely.

Using the wind information and building site, a CFD program calculated the airflow around the demonstration building, as shown in Figure 5. When wind was from the north, the air speed around the building was about 0.1 m/s ~ 0.4 m/s, which is too low to support natural ventilation of units. This reduction in airvelocity occurs because the building site is in a recirculation zone, created by the tall building to the north. When the wind was from the south, the air speed around the demonstration building was higher but still low, between 0.3 and 0.8 m/s. The small distance between the demonstration building site and adjacent buildings to the south deflects wind before it reaches the building.

Despite these conditions, the design team chose to use the available, albeit low-velocity, wind to provide natural ventilation to the building. Since the design of natural ventilation does not usually increase building construction and operation costs, it is always worthwhile to try to incorporate such systems, when basic climatic conditions will permit thermally comfortable through-ventilation. Figure 6 shows two design schemes for a typical floor of the demonstration building, both designed to allow free passage of wind. This chapter demonstrates how the CFD technique is used to determine the necessary size of the court to the south. The project team expects that the court will increase natural light in the building, create a social space for the building residents, and channel wind from the south for ventilation.

Figure 7 shows the airflow distribution in and around the demonstration building when the wind comes from the south. The separation of detailed airflow information within the demonstration building from the airflow around Beijing City Garden (Figure 5b) reduced computing time significantly. Because wind speed from the south around the demonstration building is 0.3 ~ 0.8 m/s, CFD models of airflow within the building assumed wind from the south at a uniform speed of 0.5 m/s. The model shows that the building layout permits free, easy ventilation of the units. However, court size has no significant Impact on the airflow pattern and flow rate. Furthermore, the results suggest that mechanical ventilation or stack ventilation might be needed In order to enhance the ventilation In the building. Chapter 5 documents the Impact on thermal comfort of specified air change rates assisted by mechanical ventilation. With an open Interior design for natural ventilation, fan power for adequate stack ventilation will be keptto a minimum. These results were Important In refining the final ventilation design. See Chapter 10, Case Study One -Beijing Prototype Housing for more Information.

Outdoor Comfort and Site Planning

A second example demonstrates how architects and engineers can work together to design a comfortable outdoor and Indoor environment. Acceleration of airflow among high-rise buildings may create outdoor comfort problems. This study uses the Beijing Star Garden Project as an example of how to design a comfortable outdoor environment.

Because It is mainly the chilling effect of the wind In the winter that causes the outdoor discomfort problem, the design analyzed the airflow distribution on a winter day. The wind rose for Beijing (Figure 3) illustrates that in the winter the prevailing wind Is from the north (5° inclined to the west). In a typical year, there are nine days during which the wind speed Is higher than 7.6 m/s in Beijing (ASHRAE 1997), and high wind days generally occur In the winter. The present investigation studied a scenario with a north wind of 7.6 m/s for outdoor thermal comfort consideration.

Figure 5a The airflow distribution around the demonstration building section with a north wind; existing tall buildings shown on the left-hand side of the figure

srFw-n demonstration building

Figure 6 A typical floor plan of the demonstration building with different court sizes

Figure 7 The air velocity in and around the demonstration building at a height of 1.2 meters above the floor: (a) shows air velocity in the plan with the large court, and (b) shows a similar plan with a smaller court